# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: USCG Student Example for Low Altitude
From: Antoine Couëtte
Date: 2013 Apr 27, 12:49 -0700

RE : http://fer3.com/arc/m2.aspx/USCG-Student-Example-for-Low-Altitude-Tschudin-apr-2013-g23708

Hello Marcel,

In the book "Introduction aux Ephémérides Astronomiques" published by the Bureau des Longitudes sub-chapter 7.3 deals with refraction from pages 190 to 207.

I own the (first) 1997 Edition ISBN 2-86883-298-9 , Editions de Physique. I would think that a second edition recently came out of the press.

A significant part of sub-chapter 7.3 is devoted to the Laplace's formula (Obvious : Marquis de Laplace was French ...).

Laplace's formula is not directly applicable to very low altitudes. The book gives a table for standard refraction a low altitudes. For apparent altitude 0° it gives -32'58"

For apparent altitude 0° the French Ephémérides Nautiques indicate -33'80 for 760 mm Hg (i.e. 29.92'' Hg, or 1013.25 mb/hPa) and 10°C (50°F) . The US NA value is -34.5' for slightly different "standard" conditions (same 10°C and 1010 mb as I can deduct from Table A4 additional corrections).

If we reduce the US NA 0° refraction value to the same conditions as the French EN (1013.25 mb), then we would get -34'6 for US NA value. Therefore the difference between both Almanacs is actually 0'8 (and not 1' as I earlier stated from memory).

As earlier discussed in depth in NavList, Refraction at low altitudes (certainly below 10° for our CelNav applications) can be extremely variable if not unpredictable ...

refraction also definitely depends on the light color (wavelength) and some time ago I was given this as an explanation for the differences between both Almanachs : apparently they would not be using the same wavelengths at very low altitudes.

Best Friendly Salutations

Kermit
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